Beam steering for optical target identification and tracking without gimbals or scanning mirrors

ABSTRACT

A targeting and tracking apparatus and method for optical transceivers is disclosed. The tracking function is performed internally by way of translating an internal optical fiber in the focal plane of the transceiver telescope using miniature motorized translation systems and/or micro-electro-mechanical systems (MEMS). The optical design of the transceiver provides a wide field of view and a pointing and tracking field of regard that is directly proportional to the translation of the optical fiber in the focal plane of the telescope. The apparatus and method can eliminate the need for external gimballing systems and scanning mirrors, and replace the gimballed optical beam steering function with motorized translation systems and/or MEMS that consumes very little power.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication No. 60/609,420, entitled “MEMS-Based Optical CommunicationsBeam Steering Apparatus” and filed on Sep. 13, 2004, and U.S.provisional patent application No. 60/609,413, entitled “Apparatus andMethod for Free Space Optical Communications Beam Steering withoutGimbals” and also filed on Sep. 13, 2004. The entire disclosures ofthese applications are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of contract no.F29601-02-C-0021 awarded by the United States Air Force ResearchLaboratory.

BACKGROUND OF THE INVENTION

The present invention relates to the field of optical tracking, and inparticular to the field of laser target acquisition and tracking. Moreparticularly, the present invention relates to a beam pointing andtracking apparatus and method for laser target acquisition and trackingusing miniature translational and/or rotational stages includingmicro-electro-mechanical systems (MEMS).

Laser targeting and tracking systems are today employed in a vast arrayof military and civil applications, although perhaps the most importantapplication is laser-guided weapons. A laser targeting and trackingsystem generally consists of a transmitting terminal and a receivingterminal. A transmitting terminal transmits an optical signal generatedby a source that converts electrical signals to optical signals fortransmission out of the transmitting telescope. The receiving terminalreceives the laser illuminated target “signature” signal into areceiving telescope, which focuses the optical signal into an opticalphotodetector, and then converts the light energy into an electricalsignal.

Pointing the beam that exits an optical transmitter is typically carriedout via a motorized beam steering mirror system that guides the laserbeam through the telescope to the target. As with the transmittingtelescope, the receiving telescope also uses scanning mirrors supportedby gimbals to acquire and track the incoming optical signature. Gimbalsare used to steer the mirrors in this sort of system. A gimbal is amechanical apparatus to allow a suspended object to rotate freely alongtwo simultaneous axes, within a defined angle of view. Gimbals are wellknown in the art, having been used, for example, since at least as earlyas the sixteenth century in the suspension of maritime compasses.Accurate alignment of the laser targeting system is essential for freespace laser target tracking systems. Thus such systems must provideaccurate alignment and high angular resolution in order for the receivertelescope to efficiently collect the incoming optical beam. Conversely,the transmitter telescope must be able to accurately point its beam sothat a remotely-reflecting object can efficiently reflect the opticalsignal for the receiver photodetector.

In addition to the gimbal-based systems described above, beam steeringin optical systems may also be accomplished by other means. Inparticular, some existing non-gimballed beam-steering solutions includeacousto-optics, liquid crystals, electro-optics, micro-optics,galvanometer or magnetic mirrors, and micro-mirror arrays. These typesof systems, however, have generally proven to be unwieldy, or lack thespeed, precision, and reliability necessary for high-speed,long-distance laser target tracking. Thus the most common means for beamsteering in optical communications systems remains by the use of amotorized gimballing system. A gimballing system used for the alignmentof an optical transmitter or receiver typically moves the entiretransmitting or receiving telescope through the required field of view.

Accurate alignment of the transceiver system is essential for lasertarget tracking systems. Therefore, gimballing systems must provideaccurate alignment angular resolution in order for the receivertelescope to efficiently collect the incoming optical beam. Conversely,the transmitter telescope must be able to accurately point its beam sothat a remote-receiving terminal can efficiently collect the opticalsignal for the photodetector. Mechanical gimballing systems have beenfavored in many laser tracking systems because they can provide veryfast alignment times coupled with high angular resolution.

Gimballed beam-steering systems do, however, suffer from severalimportant disadvantages. Such systems are quite heavy due to the weightof the mechanical components, motors, and servos necessary for such asystem. While weight may not be as important a factor in the design of aland-based system, weight is of paramount importance in aircraft design,which is a critically important application for laser tracking systems.Gimballing systems are also quite bulky due to the required mechanicalcomponents, which is also a significant disadvantage in the design ofairborne systems. Finally, mechanical gimballing systems require the useof a great deal of electrical power, far more power than is typicallyconsumed by the electronics associated with an optical receiver ortransmitter system. Again, while power consumption may not be asimportant a factor in permanent ground-based systems, it is a criticallyimportant factor in airborne systems, as well as in mobile ground-basedsystems such as may be mounted on land vehicles.

What is desired then is a laser tracking system that provides high speedand high angular resolution, with reduced size, weight, and powerconsumption as compared to traditional gimballing systems now employedin such devices. This need is particularly acute with respect to lasertracking systems for use with laser-guided weapons.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a laser tracking apparatus andmethod that provides angular adjustment by means of the movement of anoptical fiber residing in the focal plane of the transmitting orreceiving telescope. Specifically, the invention comprises a miniaturetranslation and/or rotation device, and/or micro-electro-mechanicalsystem (MEMS), residing in the focal plane of the transmitter orreceiver telescope. The system accurately and rapidly moves the fiber,thereby providing, in a laser tracking device, a corresponding pointingangle change in the output beam, and a corresponding relative anglechange in the receiving telescope with regard to the incoming beamangle.

The present invention achieves very fast response times while carryingout angular pointing and tracking. Because the present inventionrequires only the movement of an optical fiber, it requires theconsumption of far less power than the mechanical systems that rely upongimbals. It also allows a transmitter or receiver system to beconstructed that is of much smaller size and weight compared tocomparable gimballed systems. Because the complex mechanical componentsof gimballing systems are not required, the overall cost of thetransmitter or receiver system is significantly reduced.

It is therefore an object of the present invention to provide for alaser tracking apparatus and method that achieves high speed and angularprecision without the use of gimbals.

It is a further object of the present invention to provide for a lasertracking apparatus and method that consumes relatively little electricalpower during operation.

It is also an object of the present invention is to provide for a lasertracking apparatus and method that is of a relatively small size andweight.

It is also an object of the present invention is to provide for a lasertracking apparatus and method that has a relatively low production cost.

These and other features, objects and advantages of the presentinvention will become better understood from a consideration of thefollowing detailed description of the preferred embodiments and appendedclaims, in conjunction with the drawings as described following:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a preferred embodiment of thepresent invention as employed in a Laser Tracking and Targeting Assembly(LTTA).

FIG. 2 is a diagram illustrating the operation of the LTTA for anincoming beam angle of ΔφRX ₁ and outgoing optical beam at an angle ofΔφTX ₁ defined as the respective angles away from the optical Z-axes ofthe transmitter and receiver telescopes according to a preferredembodiment of the present invention.

FIG. 3 is a diagram illustrating the operation of the LTTA for anincoming beam angle of ΔφRX ₂ and outgoing optical beam at an angle ofΔφTX ₂ defined as the respective angles away from the optical Z-axes ofthe transmitter and receiver telescopes according to a preferredembodiment of the present invention.

FIG. 4A is a side elevational view illustrating the translation of anoptical fiber in the X-Y plane at a position that provides an input (oroutput) angle along the Z-Axis of the telescope, represented by the X-Yposition of (X₀, Y₀)=(0,0) with a fiber output beam angle of Δφ=0according to a preferred embodiment of the present invention.

FIG. 4B is an end elevational view illustrating the translation of anoptical fiber in the X-Y plane at a position that provides an input (oroutput) angle along the Z-Axis of the telescope, represented by the X-Yposition of (X₀, Y₀)=(0,0) with a fiber output beam angle of Δφ=0according to a preferred embodiment of the present invention.

FIG. 5A is a side elevational view illustrating the translation of anoptical fiber in the X-Y plane at an optical fiber position thatprovides an input (or output) angle from the optical fiber of either Δφ₁A and Δφ₁ B created by the location of the optical fiber's output in thefocal plane of (X_(1A), Y_(1A)) and (X_(1B), Y_(1B)), respectively,according to a preferred embodiment of the present invention.

FIG. 5B is a side elevational view illustrating the translation of anoptical fiber in the X-Y plane at an optical fiber position thatprovides an input (or output) angle from the optical fiber of either Δφ₁A and Δφ₁ B created by the location of the optical fiber's output in thefocal plane of (X_(1A), Y_(1A)) and (X_(1B), Y_(1B)), respectively,according to a preferred embodiment of the present invention.

FIG. 6A is a side elevational view illustrating the translation of anoptical fiber in the X-Y plane at an optical fiber position thatprovides an input (or output) angle from the optical fiber of either Δφ₂A and Δφ₂ B created by the location of the optical fiber's output in thefocal plane of (X_(2A), Y_(2A)) and (X_(2B), Y_(2B)), respectively,according to a preferred embodiment of the present invention.

FIG. 6B is a side elevational view illustrating the translation of anoptical fiber in the X-Y plane at an optical fiber position thatprovides an input (or output) angle from the optical fiber of either Δφ₂A and Δφ₂ B created by the location of the optical fiber's output in thefocal plane of (X_(2A), Y_(2A)) and (X_(2B), Y_(2B)), respectively,according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, the preferred embodiment of the presentinvention may be described. The preferred embodiment is a Laser Trackingand Targeting Assembly (LTTA), but the invention is not so limited, andin fact may be put to many other logical applications as will beapparent to those skilled in the art. The LTTA of the preferredembodiment includes a transmitter telescope 1 and a receiver telescope2. In alternative embodiments, the present invention could by applied toa transmitter-only or receiver-only arrangement, or alternatively to asystem with a transceiver arrangement, that is, a single telescope usedfor both transmitting and receiving a signal, as are known in the art.The preferred embodiment further includes fiber optic and MEMS devicepower connectors 3 and 5, for the transmitter and receiver sections,respectively. The preferred embodiment is operable to generate and senda laser signal into transmitter telescope 1 for transmission attransmitted beam 4, and is operable to receive a laser signal atreceiver telescope 2 in the form of received beam 6. In communicationwith transmitter telescope 1 and receiver telescope 2 are transmitterMEMS beam steering module 7 and receiver MEMS beam steering module 8,respectively. Module 7 is operable to translate an optical fiber passingthrough connector 3 in the X-Y plane, and module 8 is operable totranslate an optical fiber passing through connector 5 in the X-Y plane,as will be described hereafter. The LTTA, in the preferred embodiment,contains the various transmit, receive, acquisition, and trackingcontrol electronics (not shown) necessary for operation of thetransceiver, which preferably are contained in a remote electronics bay.These electronic components provide control of the required azimuth andelevation range of motion and tracking slew rate for the MEMStranslation devices to track and point the incoming and outgoing opticalbeams.

The preferred embodiment utilizes separated telescopes for optical noiseisolation; modules 7 and 8 may, however, be utilized in othertransceiver systems that require pointing and tracking. As with mostfree space optical transceiver systems, the receiver telescope of thepreferred embodiment is equipped with optical filters in order to filterout optical noise. In addition, the LTTA includes an optical lens designthat focuses the optical energy into the focal plane of receivertelescope 2. Receiver optical fiber 26A is automatically located at thefocal spot in order to collect the incoming optical signal 6. It may benoted that any selected communications band in any of the opticaldomains may be utilized; however, in the preferred embodiment opticalfiltering is utilized, providing narrow-band optical intensity, andthereby providing low noise signal detection. The system may alsoutilize a direction of arrival detection system, which in the preferredembodiment uses a charge-coupled-device (CCD) array that allows for thelocation of the angle of arrival.

FIG. 2 illustrates a preferred embodiment with the incoming receivedbeam 9 and outgoing transmitted beam 17 in a first exampleconfiguration. The configuration shows the angular direction of thebeams 9 and 17 with respect to the relative position of the LTTA.Received signal 9 arrives from a remote transmitter or target. Receivedsignal 9 arrives in an expanded form, such that only a portion ofreceived signal 9 is actually captured by receiver telescope aperture 10and focused by receiver focal plane 11 through receiver lens system 14.The received beam angle of arrival 12 determines the location of thefocused spot at receiver focal plane 11, as shown. Angle 12, which mayalso be designated as ΔφRX ₁ for purposes herein, may be defined as theangle that received signal 9 makes with respect to receiver telescopeZ-Axis 13. Coordinate axis 20 of FIG. 2 may be used as a reference forcoordinates as referred to herein.

As already described, received beam angle of arrival 12 determines thecoordinates of receiver focal plane 11 at which the optical energy willbe focused. Receiver optical fiber 26A (shown in FIG. 1) is then movedsuch that its end is aligned with that location on focal plane 11, whichas already explained is the location at which the optical energy isfocused, for purposes herein to be designated to be at X-Y coordinatesXRX ₁ and YRX ₁. The receiver optical fiber thus can receive the lightenergy being directed upon focal plane 11 at X-Y coordinates XRX ₁ andYRX ₁. The transmitter optical fiber is simultaneously moved tocorresponding transmitter focal plane location 15, defined as X-Ycoordinates XTX ₁ and YTX ₁. Light from the optical fiber 26B attransmitter focal plane 15 passes through transmitter lens system 16,which expands and collimates transmitted beam 17 in order to producediffraction limited beam propagation with minimal wave front distortionin the resulting transmitted beam 17. As may be seen, this re-locationof the transmitter optical fiber 26B to transmitter focal plane 15results in transmitted beam angle 19 (also referred to herein as ΔφTX ₁)between transmitted signal 17 and transmitter telescope Z-axis 18. Thustransmitter telescope 1 is automatically adjusted to emit a transmittedsignal 17 that is directed toward the source of received signal 9. Itmay be seen that in the preferred embodiment, received beam angle 12 andtransmitted beam angle 19 (that is, angles ΔφRX ₁ and ΔφTX ₁,respectively) are equal.

FIG. 3 illustrates a second configuration for the preferred embodimentof the present invention, with different beam angles possiblyrepresenting either a different remote transceiver terminal or a newrelative location of a remote transceiver with respect to the LTTAposition. Because of the change in location, modules 7 and 8 will movethe optical fibers 26A and 26B to this corresponding new location in thefocal planes of the respective telescopes. As in the firstconfiguration, the second configuration accepts received signal 9arriving from a remote transmitter and focuses that portion that entersthe receiver, received portion 10, onto receiver focal plane location 11through receiver lens system 14. The received beam second angle ofarrival 22 determines the coordinates of receiver focal plane location11, as shown. Received beam second angle 22, which may also bedesignated as ΔφRX ₂ for purposes herein, may be defined as the anglethat received signal 9 makes with respect to receiver telescope Z-Axis13. The receiver optical fiber 26A (shown in FIG. 1) is then moved suchthat its end is aligned with receiver focal plane 11 at which theoptical energy is focused, for purposes herein designated to be at X-Ycoordinates XRX ₂ and YRX ₂. The transmitter optical fiber issimultaneously moved to transmitter focal plane 15, defined as X-Ycoordinates XTX ₂ and YTX ₂. Light from the transmitter optical fiber26B passes through transmitter lens system 16, which expands andcollimates the optical signal to produce transmitted signal 17. As maybe seen, this re-location of the transmitter optical fiber 26B resultsin transmitted beam second angle 25 (also referred to herein as ΔφTX ₂)between transmitted signal 17 and transmitter telescope Z-axis 18. Thustransmitter telescope 1 is automatically adjusted to emit a transmittedsignal 17 that is directed toward the source of received signal 9. Itmay be seen that in the preferred embodiment, received beam second angle22 and transmitted beam second angle 25 (that is, angles ΔφRX ₂ and ΔφTX₂, respectively) are equal.

FIGS. 4A, 4B, 5A, 5B, 6A, and 6B illustrate the operation of the MEMSmodules 7 and 8 to a preferred embodiment of the present invention, byshowing the relative location and movement of optical fiber 26. (Itshould be noted here that optical fiber 26 may be situated in the focalplane of either a transmitter or receiver telescope, the distinctionbetween which is not relevant to the following discussion.) Opticalfiber 26 is fed into open fiber feed-through tube 27 through fibersupport block 28. The open tube is enclosed with the interface block 29and MEMS substrate 30. The output portion of optical fiber 26 issupported by MEMS system optical fiber support 31, with the face of thefiber exposed for transmission and/or reception of the optical beam 32into or out of the appropriate telescope. Optical beam 32 will divergeupon leaving the end of optical fiber 2, forming signal cone 34. TheMEMS translation actuation devices 33 move the fiber in the X-Y plane,as illustrated in each of the cases depicted in FIGS. 4B, 5B, and 6B,thereby moving the position of fiber optic 26 and the resultingdirection of signal cone 34. Although an infinite number of possibleoptical fiber 26 positions exist in order to achieve proper alignment toeither send or receive an optical signal, three positions will be shownand described for purposes of illustration.

FIGS. 4A and 4B illustrate Fiber Position 0, representing a positionwherein optical fiber 26 lies along the instrument Z-axis 40, and thushaving a position defined as X=0 and Y=0 in the X-Y plane. Theinput/output angle Δφ, defined as the angle formed between instrumentZ-axis 40 and the direction of radiation emitted from optical fiber 26,is zero in Fiber Position 0. As may be seen from FIG. 4B, each of MEMStranslation devices 33 are extended at equal lengths towards opticalfiber 26.

FIGS. 5A and 5B illustrate Fiber Position 1. As shown in FIG. 5A,optical fiber 26 is now below instrument Z-axis 40. FIG. 5B shows twopossible sub-configurations corresponding to Fiber Position 1,designated as Fiber Position 1A and Fiber Position 1B. In Fiber Position1A, shown in the left portion of FIG. 5B, optical fiber 26 has moved tothe lower left as viewed from the front of the device, with coordinatesdesignated as X₁ A and Y₁ A, for an input/output beam angle 41 fromoptical fiber 26 designated as Δφ₁ A. In Fiber Position 1B, shown in theright portion of FIG. 5B, optical fiber 26 has moved to the lower rightas viewed from the device, with coordinates designated as X₁ B and Y₁ B,for an input/output beam angle 41 from optical fiber 26 designated asΔφ₁ B.

FIGS. 6A and 6B illustrate Fiber Position 2. As shown in FIG. 6A,optical fiber 26 is now above instrument Z-axis 40. FIG. 6B shows twopossible sub-configurations corresponding to Fiber Position 2,designated as Fiber Position 2A and Fiber Position 2B. In Fiber Position2A, shown in the left portion of FIG. 6B, optical fiber 26 has moved tothe upper left as viewed from the front of the device, with coordinatesX₂ A and Y₂ A, for an input/output beam angle 42 from optical fiber 26designated as Δφ₂ A. In Fiber Position 2B, shown in the right portion ofFIG. 6B, optical fiber 26 has moved to the upper right as viewed fromthe device, with coordinates designated as X₂ B and Y₂ B, for aninput/output beam angle 42 from optical fiber 26 designated as Δφ₂ B.

MEMS translation devices 33 preferably provide a tracking bandwidth ofup to 10,000 Hz or greater for closed loop control. The design andconstruction of MEMS translation devices 33 is set forth in a co-pendingpatent application filed by the inventors hereof and entitled“MEMS-Based Optical Communications Beam Steering Apparatus,” the entiredisclosure of which is incorporated herein by reference. A controlsystem may be implemented to manipulate MEMS translation devices 33 inaccordance with the preferred embodiment of the present invention. Theoperation of the control loop is preferably based upon a maximization ofthe optical power collected by the receiver version of optical fiber 26,and manipulation of the transmitter version of optical fiber 26 inaccordance with its position. Various such algorithms are known in theart. In the preferred embodiment, such a control system may beimplemented in software using a microprocessor in communication with theLTTA.

The present invention has been described with reference to certainpreferred and alternative embodiments that are intended to be exemplaryonly and not limiting to the full scope of the present invention as setforth in the appended claims.

1. An optical targeting and tracking apparatus, comprising: (a) anoptical fiber comprising a first end and a second end; (b) at least oneof an optical signal receiver and an optical signal transmitter incommunication with said first end of said optical fiber; and (c) anactuator in communication with said optical fiber, wherein said actuatoris operable to deflect said second end of said optical fiber.
 2. Theapparatus of claim 1, further comprising a plurality of actuators incommunication with said optical fiber.
 3. The apparatus of claim 2,wherein each of said plurality of actuators comprises: (a) an arm; and(b) a linkage pivotally in communication with said arm and said opticalfiber.
 4. The apparatus of claim 3, wherein each of said actuator armsare at least one of retractable and extendable to deflect said secondend of said optical fiber.
 5. The apparatus of claim 4, furthercomprising an optical fiber support annularly connected to said opticfiber and pivotally in communication with said plurality of actuatorlinkages.
 6. The apparatus of claim 2, wherein said actuators areimplemented as one of miniature motorized translation systems and MEMSdevices.
 7. The apparatus of claim 2, further comprising an opticaltelescope opposite the second end of said optical fiber.
 8. A method oftracking an object using optical signals, comprising: (a) detecting anincoming optical signal from the object; (b) deflecting a receiveroptical fiber comprising a first end and a second end such that thesecond end of the receiver optical fiber is pointed toward a perceivedmaximum intensity area of the incoming optical signal, wherein saiddeflecting step is performed by means of an actuator in communicationwith the receiver optical fiber; (c) receiving at the second end of thereceiver optical fiber the incoming optical signal, and passing theincoming optical signal to the first end of the receiver optical fiber;(d) converting the incoming optical signal to an electrical signal; (e)formulating an electrical response signal to the incoming opticalsignal; (f) converting the electrical response signal to an outgoingoptical signal; (g) determining a direction for the transmission of theoutgoing optical signal based on the direction from which the incomingoptical signal was received; (h) deflecting a transmitter optical fibercomprising a first end and a second end such that the second end of thetransmitter optical fiber is pointed in the direction for thetransmission of the outgoing optical signal, wherein said deflectingstep is performed by means of an actuator in communication with thetransmitter optical fiber; and (i) receiving at the first end of thetransmitter optical fiber the outgoing optical signal, and passing theoutgoing optical signal to the second end of the transmitter opticalfiber.
 9. The method of claim 8, wherein said step of determining adirection for the transmission of the outgoing optical signal based onthe direction from which the incoming optical signal was receivedcomprises the determination of that direction that is opposite of thedirection from which the incoming optical signal was received.
 10. Themethod of claim 9, wherein said deflecting steps are performed by meansof a plurality of actuators in communication with the receiver opticalfiber and the transmitter optical fiber.
 11. The method of claim 10,wherein each of the plurality of actuators comprises an arm and alinkage pivotally in communication with the arm and a respective one ofthe receiver optical fiber and the transmitter optical fiber, andwherein said deflecting steps are performed by means of at least one ofretracting and extending at least one of the actuator arms.
 12. Themethod of claim 11, wherein at least one of the actuators areimplemented as one of motorized translation systems and MEMS devices,and said deflecting steps comprise the step of sending an electricalsignal to at least one of said actuators.
 13. An optical trackingsubsystem, comprising: (a) an optical fiber; and (b) one of a motorizedtranslation system and MEMS device in communication with said opticalfiber, wherein said one of a motorized translation system and MEMSdevice comprises at least one actuator connected to said optical fiberand operable to manipulate said optical fiber in at least one dimension.14. The subsystem of claim 13, further comprising a plurality ofactuators in communication with said optical fiber operable tomanipulate said optical fiber in at least two dimensions.
 15. Theapparatus of claim 14, wherein each of said plurality of actuatorscomprises an arm in linkage with said optical fiber.
 16. The apparatusof claim 15, wherein said one of a motorized translation system and MEMSdevice comprises at least one of retraction means and extension meansfor each of said actuator arms.
 17. The apparatus of claim 16, furthercomprising an optical fiber support annularly connected to said opticfiber and pivotally in communication with said plurality of actuatorarms.
 18. The apparatus of claim 14, further comprising a focal planeadjacent to an end of said optical fiber.
 19. The apparatus of claim 18,further comprising a telescope in communication with said focal plane,wherein an optical signal may be one of received and transmitted betweensaid focal plane and said telescope.
 20. The apparatus of claim 19,wherein said telescope comprises optical filters.